Among means for obtaining secondary electron images by a scanning electron microscope (hereinafter called “SEM”), this invention relates to a method for forming images by utilizing gas ionization and multiplication. Secondary electrons are multiplied in residual gas molecules around, then ionized gas ions are detected. The invention relates to an electrode which provide electrostatic field and an electrode which is utilized for ion current detection for improving quality of the secondary electron images during high speed scanning acquired by SEM.
Various methods for multiplying the secondary electrons by utilizing gas ionization and multiplication and then detecting the ion current have been attempted as image formation methods in SEM. The content of the principle of this kind is described in a paper “Secondary Electron Imaging in the Variable Pressure Scanning Electron Microscope”, Scanning 20, 436-441 (1998).
As a concrete application system of this means, JP-A-2001-126655 discloses a detection system that uses a sheet-like electrode as a secondary electron collector electrode during high vacuum secondary image observation and also as an electrode which provides electrostatic field (referred as “an electric field supply electrode” hereinafter) and uses a sample holder as an electrode which is used for ion current detection (referred as “an ion current detection electrode” hereinafter). JP-A-2003-132830, on the other hand, discloses a detection system that uses a secondary electron collector electrode as an electric field supply electrode and a dedicated ion current detection electrode separate from a sample holder as an ion current detection electrode. JP-A-2002-289129 discloses a detection system that uses a sample holder as both of the electric field supply electrode and the ion current detection electrode.
FIG. 8 shows a basic construction of a detection system of SEM in JP-A-2003-132830. A primary electron beam 2 is converged by an objective lens 6 onto a sample 14 put inside a gas atmosphere and a deflector 4 two-dimensionally scans the sample 14. Secondary electrons 18 are generated from the sample 14 with the progress of irradiation of the primary electron beam 2. The secondary electrons 18 are accelerated in a direction toward an electric field supply electrode 23 by the electric field generated by this sheet-like electric field supply electrode 23 to which a positive voltage is applied. The secondary electrons 18 thereby accelerated collide with gas molecules around the sample and form electron-ion pairs (gas ionization). The secondary electrons 18 and the electrons generated by ionization are further accelerated by the electric field generated by the electric field supply electrode 23, again collide with the gas molecules and form the electron-ion pairs. As this process is repeated, the number of electrons and the number of ions increase exponentially as they come close to the electric field supply electrode 23 (gas multiplication). The scale of this gas multiplication is generally great when the drift distance of the secondary electrons (distance between sample and electric field supply electrode) is great. The ions drift towards the ion current detection electrode 22 that is electrically connected to the sample holder 16 or is electrically insulated from the sample holder 16. The drifting ions are detected as an ion current. The resulting ion current signal is passed through an amplifier 19 and an A/D converter 31 and is used for image formation. The electric field supply electrode 23 shown hereby is used as a secondary electron collector electrode at the time of observation of high vacuum secondary electron images.
FIG. 9 is an enlarged view of an electric field supply electrode 23 and a sample holder 16 of an electron microscope in JP-A-2001-12655. The secondary electrons 18 emitted from the sample surface create a large number of ions due to gas multiplication in the proximity of the electric field supply electrode 23. The resulting ions 13 drift towards the sample holder 16 (ion current detection electrode) to which a ground potential or a negative voltage is applied, and are detected as an ion current from the sample holder 16. The current signal so obtained is passed through an amplifier 19 and an A/D converter 31 and is used for image formation. The electric field supply electrode 23 hereby shown is used also as a secondary electron collector electrode at the time of observation of vacuum secondary images.
FIG. 10 is an enlarged view of portions in the proximity of a sample holder 16 of an electron microscope in JP-A-2002-289129. The secondary electrons 18 emitted from the sample surface drift towards an objective lens 6 kept at a ground potential by the electric field created by the sample holder 16 to which a negative voltage is applied, and generate a large number of ions in the proximity of the objective lens 6 by gas multiplication. The resulting ions are detected as an ion current from the sample holder 16. The resulting current signal is passed through an amplifier 19 and an A/D converter 31 and is used for image formation.